The present invention relates to gas laser apparatus emitting ultraviolet radiation. More particularly, the present invention relates to gas laser apparatus, i.e. KrF excimer laser, ArF excimer laser, and fluorine laser, which perform a lasing operation with an extended oscillation pulse width.
With the achievement of small, fine and high-integration semiconductor integrated circuits, it has been demanded that projection exposure systems for the manufacture of such highly integrated circuits be improved in resolution. Under these circumstances, the wavelength of exposure light emitted from light sources for lithography is becoming shorter, and gas laser apparatus emitting ultraviolet radiation are promising as next-generation light sources for semiconductor lithography, i.e. KrF excimer laser of wavelength 248 nm, ArF excimer laser of wavelength 193 nm, and fluorine laser of wavelength 157 nm.
In these excimer laser apparatus, a laser gas is sealed in a laser chamber under several hundred kPa. That is, in the KrF excimer laser, a mixed gas of fluorine (F2) gas, krypton (Kr) gas and a rare gas, e.g. neon (Ne), as a buffer gas is sealed in the laser chamber as a laser gas. In the ArF excimer laser, a mixed gas of fluorine (F2) gas, argon (Ar) gas and a rare gas, e.g. neon (Ne), as a buffer gas is similarly sealed in the laser chamber as a laser gas. In the F2 laser, a mixed gas of fluorine (F2) gas and a rare gas, e.g. helium (He) or/and neon (Ne), as a buffer gas is similarly sealed in the laser chamber as a laser gas. In these apparatus, the laser gas as a laser medium is excited by generating an electric discharge in the laser chamber.
In these gas laser apparatus, the oscillation pulse width (Tis) is about 20 ns at maximum. Therefore, the peak power of the output light is large. In addition, because the wavelength of the output light is short, the photon energy is high. Accordingly, the probability of occurrence of two-photon absorption is higher than in the case of mercury lamps, which are conventional light sources for lithography. For this reason, the optical elements of the projection exposure system may be damaged by a compaction (an increase in refractive index), etc., causing the performance of the projection exposure system to be degraded. The oscillation pulse width (Tis) is defined byTis=[∫P(t)dt]2/∫P2(t)dt                 where P(t) is the laser intensity depending upon time t.        
To avoid the above-described problem, therefore, it is demanded that the oscillation pulse width be extended (i.e. a longer pulse width should be achieved) without a change in energy per pulse, thereby reducing the peak power of the output light. For example, a pulse width of 30 ns or longer is demanded.
There has heretofore been no particular proposition concerning an exciting circuit for realizing a longer pulse width in the above-described gas laser apparatus. However, there have been some propositions regarding the achievement of a longer pulse width in an excimer laser apparatus having a laser medium different from those mentioned above.
It is generally known that the discharge current flowing between the main discharge electrodes of an excimer laser apparatus is an oscillating current, and as shown in the waveform chart of FIG. 8, the first half-cycle of the oscillating current contributes to the laser oscillation [see “Excimer Laser” (first edition), edited by Mitsuo Maeda, p. 64, Japan Scientific Societies Press, Aug. 20, 1983].
The conventional technique intends to extend the pulse width of the above-described first half-cycle for the purpose of achieving a longer pulse width. For example, Japanese Patent Application Unexamined Publication (KOKAI) No. Sho 62-2683 discloses an example in which, as shown in FIG. 9, an inductance L, is added to one of the main discharge electrodes in the exciting circuit of a XeCl excimer laser. In addition, “The Review of Laser Engineering” Vol. 15, No. 7, pp. 63-72, shows an example in which a PFN (Pulse Forming Network) is used in the exciting circuit of a XeCl excimer laser.
It is conceivable that the application of the above-described conventional technique to the KrF excimer laser, ArF excimer laser and fluorine laser also allows realization of a longer pulse width of pulsed laser light emitted from the laser apparatus.
From the viewpoint of improving the throughput of lithographic processing for the manufacture of semiconductor devices, it has recently been demanded that excimer laser apparatus for lithography perform a high-repetition rate oscillating operation, i.e. 2 kHz or more. If it is intended to realize a longer pulse width in such a high-repetition rate oscillating operation with the conventional technique wherein an inductance is added to the main discharge circuit as shown in FIG. 9, the laser oscillating efficiency becomes extremely low. It is actually difficult with the conventional technique to realize a longer pulse width in the demanded high-repetition rate oscillating operation.